No, it's something new PBS implemented. Threads are black if you've posted in them in the last week. There's also a new column called My Last Post that you can click to go directly to your last post in that thread.

It's much better than my old method, which involved going to my profile, clicking on the number under Forum Posts, and selecting the desired post by clicking View In Line. It also makes it easier to tell whether or not anyone else has posted after me.